Technical Abstract:
Russian knapweed is an aggressive, noxious weed that dominates areas by forming monocultures. Development of Russian knapweed monocultures can be partly explained by allelopathy. Russian knapweed exudes polyacetylenes from its roots throughout the growing season and these compounds have been shown to inhibit the growth of certain plant species. A field experiment was established to determine the influence of Russian knapweed soil allelochemicals in situ on the seedling growth of lettuce, blue grama, western wheatgrass, prairie junegrass, and sand dropseed. The experiment was a randomized complete block design with 4 replications. Each transect included 10 seedlings of each bioassay species transplanted inside and directly outside a dense stand of Russian knapweed. Seedlings were irrigated over the course of 15 days. Seedling vigor was ranked visually at 3-day intervals. Blue grama survival inside the stand was 30% less than outside 3 days after the experiment began. Survival of prairie junegrass, the stand, sand dropseed, western wheatgrass, were less inside compared to outside the stand. A greenhouse experiment was also completed to assess allelopathic potential of Russian knapweed infested soils at three soil depths, (0 to 5, 5 to 10, and 10 to 15 cm). The greenhouse experiment was a randomized complete block design with 4 replications. Response variables, including plant height, number of leaves/tillers, and a visual estimate of vigor, were measured at the onset of the experiment and after 1, 3, and 5 weeks. Differences were not detected between the treatments. However, volunteer seedling emergence was significantly lower for soil from inside the stand. The results will be used to develop an allelopathic parameter for a computer simulation model of Russian knapweed invasion dynamics.